1. Field of the Invention
This invention relates to rotary helical screw compressors and more particularly to the use of slide valves for controlling compressor capacity and the discharge pressure of the machine.
2. Description of the Prior Art
Rotary helical screw compressors constitute positive displacement machines, wherein a working fluid is trapped within the closed threads of helical screw rotors whose grooves and lands are intermeshed: the screw rotors being mounted for rotation within intersecting bores with coplanar axes defining the barrel portion of a screw compressor casing. In order to control the capacity of the compressor and to control the pressure ratio or the pressure of the working fluid at compressor discharge, slide valves have been provided to the compressor which are carried within axially extending recesses within the barrel portions of the casing in open communication with the bores and to respective sides of the intermeshed screws. U.S. Pat. No. 3,088,659 to H. R. Nilsson et al and entitled "Means for Regulating Helical Rotary Piston Engine" is exemplary of the employment of such slide valves within rotary helical screw compressors.
In an effort to improve lubrication and cooling of the parts of the helical screw compressor forming the compressor working chamber, attempts have been made to inject liquid refrigerant, water, oil, and relatively low temperature gas into a closed thread of the compressor by means of a port, carried by the slide valve and opening up into the compressor working chamber upstream of the discharge port of the screw compressor and movable with the slide valve to shift the injection port automatically with the shift of the slide valve, which controls the machine capacity by bypassing a portion of the compressed working fluid near the suction side of the machine, back to the suction port. Such liquid refrigerant injection is the subject matter of U.S. Pat. No. 3,795,117 to Moody et al entitled "Injection Cooling of Screw Compressors".